Organic light emitting materials as well as light emitting devices containing these materials

ABSTRACT

The present invention relates to an electro-luminescent material, which is characterized in that it comprises a liquid crystalline mixture containing at least one liquid crystalline compound with a negative dielectric anisotropy, to an electro-luminescent device containing such an electro-luminescent material as well as to a method of preparation such a device.

FIELD OF THE INVENTION

The present invention relates to organic light emitting materials andlight emitting devices containing these materials as well as theirfabrication processes. In particular, the present invention relates toorganic light emitting materials which emit polarized light and lightemitting devices with such materials as well as their fabricationprocesses. The light emitting devices are particularly suitable for backlight units in liquid crystalline displays.

PRIOR ART

Inorganic light emitting diodes have been frequently used as lightemitting devices. Two classes of these inorganic materials, the one,such as ZnS, ZnSe and ZnS:Mn, which glow by accelerated charge carriers'collision with emission centers, and the other, such as GaN, SiC andGaAs, which glow by recombination of the charge carriers injectedthrough the electrodes, are used as flat electro-luminescent displaysand flat lamps.

Tang et al. describe an electro-luminescent (abbreviated as EL from nowon) device using evaporated organic amorphous films (Applied PhysicsLetters, Vol. 51, P. 913 (1987)). Such organic EL materials areextensively developed and are now used in indicators for car use, thoughthey can only display monochromatic light.

Furthermore, methods which give polarized light emission are describedin Japanese Patent 1996-306954. In these methods, rubbed π-conjugatedpolymers or π-conjugated polymers on rubbed orientation layers are usedas light emitting layers. Other methods which give polarized lightemission are described in WO 97/07654. These methods comprise the stepsof coating a mixture of liquid crystalline (abbreviated as LC from nowon) compounds and organic EL materials and after aligning the mixture,UV light is irradiated to polymerize the LC materials, and hence, to fixthe compounds' orientation which mixture shows polarized light emission.It is also described in Japanese Patent 1999-241069 that polarized lightemission is achieved by applying a high voltage to a smectic liquidcrystalline phase of naphthalene derivatives or biphenyl derivativesincorporated in fluorescent dye materials.

Problems to be Solved

However, the inorganic materials have difficulties in that they requirequite a high driving voltage and in that the luminous efficiency forblue is not sufficient. Moreover, it is extremely difficult to getpolarized light emission with inorganic materials. As for the evaporatedorganic films, it is difficult to obtain a uniform film for a large areaand the film quality is so sensitive to the fabrication process that theprocess window becomes very small. It is also quite difficult to achievepolarized light emission for amorphous organic films.

For π-conjugated polymers, the rubbing process orients only a very thinsurface layer of the polymer film. On the rubbed orientation layer,π-conjugated polymers align toward the rubbing direction above theirglass transition temperature, however, this process is very slow due totheir high viscosity, and the resulting orientation order is mostly verypoor.

Liquid crystalline materials improve the orientation difficulty,however, there still remains the problem that, upon applying theelectric field, the long axis of the liquid crystalline molecules tendsto align along the electric field, which orientation is very detrimentalto an electric charge hopping mechanism among π-conjugated systems andrestricts the electronic charge transport through the medium. Because ofthis phenomenon, for the methods in which liquid crystalline materialsare polymerized to fix their orientation, one cannot utilize theelectric field to facilitate better molecular orientation, and moreover,extensive polymerization is necessary which in fact results in that lessfunctional compounds, such as dyes and electron or hole transportingmaterials, are available. As for the methods in which the smectic phaseof the naphthalene derivatives or the biphenyl derivatives is used, inorder to suppress the molecular orientation change by the electricfield, a high viscous smectic phase is necessary which limits the kindof materials, the temperature range and the process windows.

THE PRESENT INVENTION

According to the present invention there is provided anelectro-luminescent material which is characterized in that it comprisesa liquid crystalline mixture containing at least one liquid crystallinecompound with a negative dielectric anisotropy.

In a preferred embodiment, the liquid crystalline compound thatpossesses a negative dielectric anisotropy is present in theelectro-luminescent material in an amount sufficient that the wholeelectro-luminescent material shows a dielectric anisotropy which is ≦0.As a result thereof, the inconvenient state, according to which the longaxis of the molecules aligns along the electric field, never occurs.Even if the content of the liquid crystalline compound with the negativedielectric anisotropy is not sufficient to cancel the positivedielectric anisotropy completely, it can diminish the positivedielectric effect and the orientation state shifts to be muchpreferable. That is, the restriction on variety and amount of liquidcrystalline compounds and incorporating compounds is remarkablydiminished which in fact enables to use vastly different kinds ofcompounds and makes the process window quite wide. In the case of fixingmolecular orientation using polymerization, according to the presentinvention, a low cross-linking density system becomes available, sincethe inconvenient state, according to which the long axis of themolecules aligns along the electric field, does not occur, or thepositive dielectric effect is considerably diminished. Thus, the contentof the compounds without polymerizable functional group can be increasedand more variety of compounds can be utilized. The process windowbecomes wider, too, since controlling the cross-linking density is notvery strict. Moreover, alignment becomes better, since, for the systemwith a negative dielectric anisotropy, the electric field helpsmolecules to align in the preferable direction besides alignment layer.

According to the present invention, the compound with a negativedielectric anisotropy can take any structure as long as it has a strongelectronegative group, such as cyano group and halogen, along its shortaxis direction.

In a preferred embodiment, the liquid crystalline compound with anegative dielectric anisotropy contains at least one of the followingunits a to f:

wherein Hal is fluorine, chlorine or bromine, preferably fluorine.

The compounds with the structure a, preferably with Hal is fluorine, areparticularly preferred, due to their stability and compatibility.

According to the present invention, the liquid crystalline mixtures thatcontain at least one liquid crystalline compound with a negativedielectric anisotropy provide excellent performance as mentioned above.

Furthermore, compounds whose ionization potential is lower than 6.1 eVlower the driving voltage remarkably. As these compounds, those with atolane unit are preferred because a tolane unit can be easily introducedinto a liquid crystalline compound.

Preferred are tolanes of the formula IR¹(—A¹—Z¹)_(m)—A³—C≡C—A⁴(—Z²—A²)_(n)—R²  Iin which

-   -   R¹ and R² are identical or different and are each, independently        of one another, H, halogen (F, Cl, Br or I) or a linear or        branched, optionally chiral alkyl or alkoxy radical having from        1 to 15 carbon atoms which is unsubstituted or monosubstituted        or polysubstituted by halogen and in which one or more CH₂        groups may each be replaced, independently of one another, by        —O—, —S—, —CO—, —CO—O—, —O—CO—, —O—CO—O—, —CH═CH—, —CH═CF—,        —CF═CF—, —C≡C— or —⋄— in such a way that heteroatoms are not        linked directly to one another, —CN, —SCN, —NCS, —SF₅, —SCF₃,        —CF₃, —CF═CF₂, —CF₂CF₂CF₃, —OCF₃, —OCHF₂, —CF₂CH₂CF₃ or        —OCH₂CF₂CHFCF₃,    -   A¹, A², A³ and A⁴ are identical or different and are in each        case, independently of one another,        -   a) trans-1,4-cyclohexylene, in which, in addition, one or            more non-adjacent CH₂ groups may be replaced by —O— and/or            —S— and in which, in addition, one or more H atoms may be            replaced by F,        -   b) 1,4-phenylene, in which one or two CH groups may be            replaced by N and in which, in addition, one or more H atoms            may be replaced by halogen (F, Cl, Br or I), —CN, —CH₃,            —CHF₂, —CH₂F, —OCH₃, —OCHF₂ or —OCF₃,        -   c) a radical from the group consisting of            bicyclo[1.1.1]pentane-1,3-diyl,            bicyclo[2.2.2]octane-1,4-diyl, spiro[3.3]-heptane-2,6-diyl,            naphthalene-2,6-diyl, decahydro-naphthalene-2,6-diyl,            1,2,3,4-tetrahydronaphthalene-2,6-diyl and            piperidine-1,4-diyl, or        -   d) 1,4-cyclohexenylene,    -   Z¹ and Z² are identical or different and are in each case,        independently of one another, —O—, —CH₂O—, —OCH₂—, —CO—O—,        —O—CO—, —CF₂O—, —OCF₂—, —CF₂CF₂—, —CH₂CF₂—, —CF₂CH₂—, —CH₂CH₂—,        —CH═CH—, —CH═CF—, —CF═CH—, —CF═CF—, —CF═CF—COO—, —O—CO—CF═CF—,        —C≡C— or a single bond, and    -   m and n are identical or different and are, independently of one        another, 0 or 1,        with the proviso that at least one of the groups A¹, A², A³ and        A⁴ is selected from the units a to f.

Preferred compounds of the formula I are compounds of the partialformulae Ia (having two rings), Ib to Ie (having three rings) and If toIi (having four rings):R¹—A³—C≡C—A⁴—R²  IaR¹—A¹—A³—C≡C—A⁴—R²  IbR¹—A¹—Z¹—A³—C≡C—A⁴—R²  IcR¹—A³—C≡C—A⁴—A²—R²  IdR¹—A³—C≡C—A⁴—Z²—A²—R²  IeR¹—A¹—A³—C≡C—A⁴—A²—R²  IfR¹—A¹—Z¹—A³—C≡C—A⁴—A²—R²  IgR¹—A¹—A³—C≡C—A⁴—Z²—A²—R²  IhR¹—A¹—Z¹—A³—C≡C—A⁴—Z²—A²—R²  Iiin which R¹, R², A¹, A², A³, A⁴, Z¹ and Z² are as defined above andwherein at least one of the groups A¹, A², A³ and A⁴ is selected fromthe units a to f.

Particularly preferred compounds of the formula I and formulae Ia to Iiare those in which at least one of the groups A¹, A², A³ and A⁴ is a2,3-dihalogeno-1,4-phenylene group (unit a). In this group halogen isfluorine, chlorine or bromine, preferably fluorine.

Especially preferred are compounds of formula I and formulae Ia to Ii,wherein A³ or A⁴ is a 2,3-difluoro-1,4-phenylene group.

Examples of the especially preferred representatives of compounds of theformula I and formulae Ia to Ii having a 2,3-difluoro-1,4-phenylenegroup are illustrated in the following:

Preference is given to compounds of the formula I, formulae Ia to II aswell as formulae IIa to IIi in which R¹ and R² are identical ordifferent and are each, independently of one another, a linear orbranched, preferably linear, optionally chiral alkyl or alkoxy radicalhaving from 1 to 15 carbon atoms, preferably 1 to 10 carbon atoms, whichis unsubstituted or monosubstituted or polysubstituted by halogen.

If R¹ and/or R² in the formulae above and below is an alkyl radical,this may be straight-chain or branched. It is particularly preferablystraight-chain, has 2, 3, 4, 5, 6 or 7 carbon atoms and accordingly ismethyl, ethyl, propyl, butyl, pentyl, hexyl or heptyl, furthermoreoctyl, nonyl, decyl, un-decyl, dodecyl, tridecyl, tetradecyl orpentadecyl.

If R¹ and/or R² is an alkyl radical in which one CH₂ group has beenreplaced by —O—, this may be straight-chain or branched. It ispreferably straight-chain and has from 1 to 7 carbon atoms. The firstCH₂ group of this allyl radical has particularly preferably beenreplaced by —O—, so that the radical R¹ and/or R² attains the meaningalkoxy and is, in particular, methoxy, ethoxy, propoxy, butoxy,pentyloxy, hexyloxy or heptyloxy, furthermore octyloxy, nonyloxy,decyloxy, undecyloxy, dodecyloxy, tridecyloxy, tetradecyloxy orpentadecyloxy.

A¹, A², A³ and A⁴ in formula I, formulae Ia to Ii and IIa to IIi arepreferably identical or different and are in each case, independently ofone another, a trans-1,4-cyclohexylene, in which, in addition, one ormore non-adjacent CH₂ groups may be replaced by —O— and in which, inaddition, one or more H atoms may be replaced by F, or a 1,4-phenylene,in which one or two CH groups may be replaced by N and in which, inaddition, one or more H atoms may be replaced by halogen (F, Cl or I),—CN, —CH₃, —CHF₂, —CH₂F, —OCH₃, —OCHF₂ or —OCF₃ but with the provisothat wherein at least one of the groups A¹, A², A³ and A⁴ is selectedfrom the units a to f

Preferably, Z¹ and Z² are identical or different and are in each case,independently of one another, —O—, —CH₂O—, —OCH₂—, —CO—O—, —O—CO—,—CF₂O—, —OCF₂— or a single bond, more preferably a single bond.

In particular preferred are compounds of the formulae IIa and IIbwherein R¹ is an alkyl group whose carbon number m is from 1 to 7,wherein R² is an alkoxy group whose carbon number n is from 1 to 7 andwherein Z¹ is a single bond:

because they possess a negative dielectric anisotropy, an ionizationpotential lower than 6.1 eV and compatibility to other liquidcrystalline compounds simultaneously.

In another embodiment of the present invention, the liquid crystallinemixtures contain at least one liquid crystalline compound with anegative dielectric anisotropy of formula IIIR¹(—A¹—Z¹)_(m)—A³—(Z²—A²)_(n)—R²  IIIin which

-   -   R¹ and R² are identical or different and are each, independently        of one another, H, halogen (F, Cl, Br or I) or a linear or        branched, optionally chiral alkyl or alkoxy radical having from        1 to 15 carbon atoms which is unsubstituted or monosubstituted        or polysubstituted by halogen and in which one or more CH₂        groups may each be replaced, independently of one another, by        —O—, —S—, —CO—, —CO—O—, —O—CO—, —O—CO—O—, —CH═CH—, —CH═CF—,        —CF═CF—, —C≡C— or        in such a way that heteroatoms are not linked directly to one        another, —CN, —SCN, —NCS, —SF₅, —SCF₃, —CF₃, —CF═CF₂,        —CF₂CF₂CF₃, —OCF₃, —OCHF₂, —CF₂CH₂CF₃ or —OCH₂CF₂CHFCF₃,    -   A¹ and A² are identical or different and are in each case,        independently of one another,        -   a) trans-1,4-cyclohexylene, in which, in addition, one or            more non-adjacent CH₂ groups may be replaced by —O— and/or            —S— and in which, in addition, one or more H atoms may be            replaced by F,        -   c) 1,4-phenylene, in which one or two CH groups may be            replaced by N and in which, in addition, one or more H atoms            may be replaced by halogen (F, Cl, Br or I), —CN, —CH₃,            —CHF₂, —CH₂F, —OCH₃, —OCHF₂ or —OCF₃,        -   c) a radical from the group consisting of            bicyclo[1.1.1]pentane-1,3-diyl,            bicyclo[2.2.2]octane-1,4-diyl, spiro[3.3]-heptane-2,6-diyl,            naphthalene-2,6-diyl, decahydro-naphthalene-2,6-diyl,            1,2,3,4-tetrahydronaphthalene-2,6-diyl and            piperidine-1,4-diyl, or        -   d) 1,4-cyclohexenylene,    -   A³ is selected from the units a to f,    -    Hal is fluorine, chlorine or bromine, preferably fluorine    -   Z¹ and Z² are identical or different and are in each case,        independently of one another, —O—, —CH₂O—, —OCH₂—, —CO—O—,        —O—CO—, —CF₂O—, —OCF₂—, —CF₂CF₂—, —CH₂CF₂—, —CF₂CH₂—, —CH₂CH₂—,        —CH═CH—, —CH═CF—, —CF═CH—, —CF═CF—, —CF═CF—COO—, —O—CO—CF═CF—,        —C≡C— or a single bond, and    -   m and n are identical or different and are, independently of one        another, 0, 1, 2 or 3, wherin the sum (m+n) is 1, 2 or 3.

Particularly preferred compounds of the formula III are those in whichat least group A³ is a 2,3-dihalogeno-1,4-phenylene group (unit a). Inthis group halogen is fluorine, chlorine or bromine, preferablyfluorine. Therefore, especially preferred are compounds of formula I,wherein A³ is a 2,3-difluoro-1,4-phenylene group.

Examples of the especially preferred representatives of compounds of theformula III having a 2,3-difluoro-1,4-phenylene group are illustrated inthe following:

Preference is given to compounds of the formula III as well as formulaeIIIa to IIIi in which R¹ and R² are identical or different and are each,independently of one another, a linear or branched, preferably linear,optionally chiral alkyl or alkoxy radical having from 1 to 15 carbonatoms, preferably 1 to 10 carbon atoms, which is unsubstituted ormonosubstituted or polysubstituted by halogen.

A¹ and A² in formula III and IIIa to IIIi are preferably identical ordifferent and are in each case, independently of one another, atrans-1,4-cyclohexylene, in which, in addition, one or more non-adjacentCH₂ groups may be replaced by —O— and in which, in addition, one or moreH atoms may be replaced by F, or a 1,4-phenylene, in which one or two CHgroups may be replaced by N and in which, in addition, one or more Hatoms may be replaced by halogen (F, Cl or I), —CN, —CH₃, —CHF₂, —CH₂F,—OCH₃, —OCHF₂ or —OCF₃.

Preferably, Z¹ and Z² are identical or different and are in each case,independently of one another, —O—, —CH₂O—, —OCH₂—, —CO—O—, —O—CO—,—CF₂O—, —OCF₂— or a single bond, more preferably a single bond.

In particular preferred are compounds of the formulae IIIa and IIIcwherein R¹ is an alkyl group whose carbon number m is from 1 to 7 andwherein R² is an alkyl or alkoxy group whose carbon number n is from 1to 7:

For the purpose to obtain light emission by applying voltage, it is notnecessary to align the molecular axis along a certain direction.However, even in such a case, it is important that the liquidcrystalline mixture contains at least one compound with a negativedielectric anisotropy. In contrast to amorphous materials, liquidcrystalline materials have the following advantages: They can form alarge area uniform film because of their fluidity, they do notcrystallize by Joule's heat during operation and their light emissionproperties are insensitive to defects such as domain boundary. In orderto make the best use of these advantages, it is necessary to suppressthe phenomenon that the long axis of the liquid crystalline moleculesaligns along the operating electric field and charge transport throughthe liquid crystalline medium is hindered which results in no lightemission. According to the present invention, this is easily achieved byusing the liquid crystalline mixture which contains at least onecompound with a negative dielectric anisotropy without sacrificing otherimportant properties.

For the application in back light units of liquid crystalline displaydevices, polarized light emission is preferred from the point of view ofefficiency. Molecular orientation alignment is necessary to achievepolarized light emission. As described in WO 97/07654 and JapanesePatent 1999-24069, liquid crystalline materials provide betterorientation with preferable alignment process compared with the methodsin which polymer films are elongated.

According to the present invention, the liquid crystalline mixtureswhich contain at least one compound with a negative dielectricanisotropy not only provide a better orientation but also suppress thephenomenon that the liquid crystalline molecules align along theelectric field. Thus, they have great advantage in vast choice ofcompounds and wide process window. The negative dielectric anisotropyalso allows the electric field to facilitate liquid crystallinealignment because the electric field direction and short axis of themolecules agree perfectly.

According to the present invention, a transparent electrode composed ofITO etc. is fabricated on the first transparent substrate composed ofglass, polycarbonate, polyethersulfone etc. Polyimide or its precursor,polyamic acid, is coated as an alignment layer on the first substrate,and after baking the substrate to evaporate the solvent or to facilitateimidization, the alignment layer is rubbed. Here, instead of thepolyimide a mixture of poly(styrenesulfonate) andpoly(2,3-dihydrothieno[3,4-b]-1,4-dioxin)PEDOT) can be used tofacilitate the hole injection process. The first substrate can be rubbedwithout the alignment layer depending on the materials. Metal with smallwork function is formed on the second substrate as counter electrode,and the surface is rubbed if necessary. An alignment layer such aspolyimide can also be applied on the second substrate, which alignmentlayer can be rubbed after baking. This rubbing is not definitelynecessary and can be omitted if the metal surface is easily scratched. Aphoto-alignment layer, as it is described in AM-LCD'96/IDW'96 Digest ofTechnical Papers P. 337 (1996), whose functional group polymerizesselectively upon polarized light irradiation, is preferred because itavoids mechanical rubbing, and hence, allows soft metal materials' useas electrode. Polymer or glass beads or polymer pillars that arefabricated on either substrate are used as spacer, and two substratesare put together with suitable sealing agent which is formed on eithersubstrate and polymerized using heat or UV-light. The obtained cell iscut into pieces, if necessary. The liquid crystalline mixtures thatcontain at least one compound with a negative dielectric anisotropy ismixed with the fluorescent dye and hole or electron transport materialsdepending on necessity, and then introduced into the cell either atnematic phase or isotropic phase. The introduction port is filled withUV-polymerizing agent. When plural pixels with different colors aredesirable, the sealing agent can be patterned as a desirable number ofpixels, and liquid crystalline mixtures that show each color can beintroduced from each port at the same time and/or sequentially. At asuitable temperature the system shows liquid crystalline phase and tendsto align so that the long axis of the molecules agree with the rubbingdirection. Keeping the system in the nematic phase at least at roomtemperature is most preferably for alignment. In the case that theorientation control is difficult, such as the system possesses only asmectic phase, it is recommendable to heat the system into the isotropicphase and to cool down slowly. Such a process improves the liquidcrystalline alignment considerably. According to the present invention,it is further preferred to apply an electric field during the coolingprocess because the electric field direction agrees with the short axisof the molecules. Such a geometry improves the liquid crystallinealignment. This geometry also holds good during operating voltageapplication, that is, it avoids the inconvenient phenomenon that thelong axis of the molecules aligns along the electric field and leads tono light emission as in the case of conventional liquid crystallinematerials.

According to the present invention, the above mentioned preferredeffects exist also in systems in which a polymerizable liquidcrystalline monomer material, such as a liquid crystalline diacrylate,is incorporated. After introducing the liquid crystalline materials intoa cell, the molecules are aligned in the liquid crystalline phase.During this process or cooling process, it is possible to apply anelectric field to facilitate the molecular alignment. Such a procedureimproves the liquid crystalline alignment considerably. The system alsohas a preferred geometry during operation.

According to the present invention, the following process is alsopossible. A metal or ITO electrode is formed on the second substrate andan alignment layer, such as polyimide, is coated and rubbed. A cell isfabricated with tentative sealing and the liquid crystalline mixturewith the polymerizable agent, such as liquid crystalline diacrylate, isintroduced into the cell. An electric field is applied during thecooling process in order to achieve a precisely controlled orientation.After fixing the orientation using UV-irradiation, the second substrateis removed and then the counter electrode using a small work functionmetal is formed. Here, even Li or Li-containing alloy, that is veryactive in the air, is applicable. In this case, after the electrodeformation, the cell will be covered with epoxy resin or inactive metal.This method is preferred because it enables to use a very small workfunction metal, and hence, electron injection efficiency becomes quitehigh which in fact results in a high light emission efficiency.

With respect to the energy levels for each layer material, it ispreferred that in the sequence from the hole injecting layer toward theelectron injecting layer, each layer's ionization potential becomeshigher and higher and/or each layer's energy of lowest unoccupiedmolecular orbit becomes lower and lower. Even if one of these conditionsis met, it will be effective for highly efficient charge injection, andhence, high luminescence efficiency. When a luminous material isincorporated, at least one luminous material's ionization potential ishigher than those of the neighboring layers and/or at least one luminousmaterial's energy of lowest unoccupied molecular orbit is lower thanthose of the neighboring layers. When there are two kinds of neighboringmaterial, it is desired that the above conditions are satisfied for bothmaterials, but only for one material it is effective as well. It isdesired that the above conditions are satisfied for both ionizationpotentials and the energy of lowest unoccupied molecular orbit, but onlyfor one of them it is effective as well.

PREFERRED EMBODIMENTS OF THE PRESENT INVENTION

Hereafter the present invention will be explained more precisely.

Invention Form 1

The concept of the EL device using the liquid crystalline light emittingcompounds according to the present invention is shown in FIG. 1. Sincethe system contains the compound with a negative dielectric anisotropy101, it possesses a negative dielectric anisotropy and, upon applying anelectric field, the long axis of the molecules align parallel to thesubstrate. This geometry agrees well with that geometry in whichinjected charge carriers transport quite smoothly through the liquidcrystalline medium. On the other hand, when the system does not containthe compound with a negative dielectric anisotropy, upon applying anelectric field, the long axis of the molecules align perpendicular tothe substrate. In this geometry, injected charge carriers are almostimpossible to transport through the liquid crystalline medium.

FIG. 1 shows a cross section of a luminescent device illustrating theconcept of invention form 1.

Explanation of the symbols:

101 compound with a negative dielectric anisotropy

102 fluorescent dye

103 liquid crystalline mixture

104 the first transparent substrate

105 transparent electrode

106 electrode

107 the second substrate

According to the present invention, as the compound with a negativedielectric anisotropy, in principle, any compound can be applied as longas it possesses a negative dielectric anisotropy. Preferred examples ofliquid crystalline compounds with a negative dielectric anisotropycontain at least one of the units a to f. These examples are thecompounds with groups having a high electronegativity, such as halogen,cyano group, trifluoromethyl group, attached to the benzene ring forminga certain angle against the molecule's long axis. The compounds whichhave two fluorine substituents (structure a) are particularly preferredfrom the point of view of both electronegativity strength and heat orlight stability.

According to the present invention, liquid crystalline EL materials withlow ionization potential are preferred from the point of view ofoperating voltage. The compounds whose ionization potential is lowerthan 6.1 eV are particularly preferred since they show a remarkable lowoperating voltage. As such compounds, tolane derivatives have been foundto show excellent effects, among tolane derivatives, the compounds withstructure II are particularly preferred because they possess a negativedielectric anisotropy and a potential energy lower than 6.1 eV as well.In structure I, there is no limitation in the number of carbon atoms inR, however, unnecessarily many carbon atoms are detrimental to a liquidcrystal. Therefore, the number of carbon atoms in R is preferably in therange between 1 and 7, and particularly preferred in the range between 1and 5.

According to the present invention, some liquid crystalline EL materialsemit light themselves, however, some do not emit because therecombination ratio between electrons and holes is so small or theiremission wavelength is in the UV-region. In this case a dye material canbe doped to promote the emission efficiency in the visible region.Fluorescent dye 102 are mainly used as dye material, dichroic dyes areused for polarized light emission. Applicable dyes are acridinederivatives, such as, 3,6-bis-dimethylaminoacridine, 9-aminoacridine,9-(4-diethylamino-1-methylbutylamino)-3-chloro-7-methoxyacridine,aryl-naphthalene-sulfonates, such as,N-methyl-2-anilinonaphthalene-6-sulfonate,2-p-toluidinyl-naphthalene-6-sulfonate, cyanine dyes, such as,1,1′-dihexyl-2,2′-oxacarbocyanine,3,3′-dipropylthia-dicarbocyanine(iodide),5-[(3-sulfopropyl-2(3H)-benzoxazolylidene)-2-butenylidene]-1,3-dibutyl-2-thiobarbituricacid, and other dyes, such as, tetramethyidiaminodiphenylketoiminehydrochloride, 1,6-diphenyl-1,3,5-hexatriene,2′,4′,5′,7′-tetrabromofluorescein,2,7-diamino-9-phenylphenanthrium-10-ethylbromide,9-(o-carboxyphenyl)-6-hydroxy-3H-xanthen-3-one,4-benzoylamido-4′-aminostilbene-2,2′-disulfonate,bis[3-phenyl-5-oxoisoxazol-4-yl]pentamethineoxonol,bis[1,3-dibutylbarbituric-acid(5)]pentamethineoxonol,α(9,11,13,15-cis,trans,trans,cis) octadecatetraenoic acid,β(9,11,13,15-all-trans)octadecatetraenoic acid, perylene(dibenz[de,kl]anthracene), N-phenyl-1-naphthylamine, pyrene(benzo[def]phenantherene), 2,8-dimethyl-3,7-diamino-5-phenylphenaziumchloride, 4-phenylspyro[furan-2(3H),1′-futalan]-3,3′-dione, o-phthalicdicarboxaldehyde, 1-dimethylaminonaphthalene-5-sulfonyl chloride,fluorescein isothiocyanate, 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole,N-dansyl aziridine, N-(1-anilinonaphthyl-4)maleimide,7-dimethylamino-4-methylcoumarynyl)maleimide, N-(3-pyrene)maleimide,Coumarin 6, BBOT(2,5-bis-(5-tert-butyl-2-benz-oxazolyl)thiophene),rubrene, Nile Red. Applicable dyes are not necessarily limited to thesedyes. The dyes either can be used as a single compound or several dyescan be mixed if necessary. Complexes, such as(8-hydroxyquinoline)aluminium, 3(tris(2-phenylpyridine)iridium andoligomers, such as polyfluorene and polyalkylthiophene are alsoapplicable.

According to the present invention, EL devices can be fabricated usingthe following method. A transparent electrode 105, such as ITO, isformed on the first transparent substrate 104 on which a few hundred-nmhigh pillar spacers (not shown in FIG. 1) are formed scores of μm apartfrom one after another using photo-lithography technique. There is nolimitation to the height of the pillars, from the point of view of loweroperating voltage, 50 nm to 1 μm is preferred and 100 nm to 300 nm isparticularly preferred. As for the quality of the material for thepillars, though every thing is applicable as long as it is insulating,photo-sensitive acrylate is preferably used from the point of view ofstress and processability. Beads spacers used for liquid crystallinedisplays are also applicable instead of pillar spacers. Glass is usuallyused for the transparent substrate, however, according to the necessity,plastics or plastics covered with thin glass are also applicable. Asealing agent (not shown in FIG. 1) is painted on the first transparentsubstrate 104 on which spacers have been formed. The second substrate107, on which, besides ITO, a small work function metal, such as Li—Al(Li 0.2%) alloy is formed as a counter electrode 106, is aligned withthe first transparent substrate 104 and the sealing agent is hardened byheat or UV treatment while applying pressure to keep the two substratesgap properly. A liquid crystalline mixture 103 which contains at leastone compound with a negative dielectric anisotropy and having optionallya dye material incorporated, is introduced into the above fabricatedcell and the filling port is fixed with UV polymerizable resin. Uponapplying an electric field, the incorporated dye starts light emissionabove a certain operating voltage. Since the system possesses a negativedielectric anisotropy, the molecular orientation direction by theelectric field agrees well with the molecular orientation in whichinjected charges are transported smoothly. The case that the liquidcrystalline molecules are oriented perpendicular to the substrates asshown in FIG. 2 never occurs.

FIG. 2 shows such a cross section of a luminescent device illustratingthe concept of the prior art.

Explanation of the symbols:

202 fluorescent dye

203 liquid crystalline mixture

204 the first transparent substrate

205 transparent electrode

206 electrode

207 the second substrate

Here, only sandwiching electrodes are explained. Not explained arein-plane switching electrodes, which are fabricated on the samesubstrate. However, according to the present invention, the net effectsare exactly the same for both cases.

According to the present invention, in principle, every process can beperformed in air atmosphere. In the case that a part of the liquidcrystalline compounds is easy to be oxidized or the metal electrode istoo active and quick to erode, the cell fabricating process can beperformed under inert gas atmosphere, such as nitrogen.

Invention Form 2

Another form of this invention is explained in FIG. 3. The differencebetween invention form 1 and 2 is that at least one substrate is subjectto an alignment treatment.

FIG. 3 shows a cross section of a luminescent device illustrating theconcept of invention form 2.

Explanation of the symbols:

301 compound with a negative dielectric anisotropy

302 fluorescent dye

303 liquid crystalline mixture

304 the first transparent substrate

305 transparent electrode

306 electrode

307 the second substrate

308 alignment layer

309 alignment layer

In the same way as in invention form 1, a transparent electrode 305,such as ITO, is formed on the first transparent substrate 304 on which afew hundred-nm high pillar spacers (not shown in FIG. 3) are formedscores of μm apart from one after another using photo-lithographytechnique. The electrode surface is rubbed using a cloth in order tocontrol the orientation of the liquid crystals. An alignment layer 308,composed of polyimide etc., can be coated on the first transparentsubstrate 304 and be rubbed after baking. For the alignment layer 308,besides polyimide, also a mixture of poly(styrene-sulfonate) andpoly(2,3-dihydrothieno[3,4-b]-1,4-dioxin) PEDOT) can be applied in orderto facilitate the hole injection from the transparent electrode 305. Thesurface of the second substrate 307, on which, besides ITO, a small workfunction metal electrode 306 is formed, is not necessarily rubbed. Butit may be rubbed to make the liquid crystalline alignment stronger.Photo-alignment technique is preferably applied for aligning liquidcrystals on the second substrate 307, especially, when theinconvenience, as electrode peeling or surface scratching, occurs, dueto a mechanical treatment, like rubbing on a coated alignment layer 309like polyimide, since metal is generally soft and a small work functionmetal is particularly weak against mechanical treatment. Photo-alignmenttechnique is also applicable to the first transparent substrate 304.

The first transparent substrate 304 and the second substrate 307, atleast one of both is treated for liquid crystal alignment, are alignedand fixed using an adhesive agent which is hardened either by heat or UVlight. In the same way as in invention form 1, a liquid crystallinemixture that contains at least one compound with a negative dielectricanisotropy and having optionally incorporated a dye material, isintroduced into the fabricated cell and the filling port is fixed with aUV hardening resin. The liquid crystalline compounds align along thealignment treatment direction. The cell may be heated above theisotropic temperature of the liquid crystalline compounds and cooleddown slowly in order to get a better alignment. This process eliminatesthe problem, such as flow alignment. Applying a voltage during thecooling process, if necessary, further improves the liquid crystalalignment. Here, according to the present invention, since the systempossesses a negative dielectric anisotropy, long axis of the moleculesalign parallel to the substrate surface, and such an inconvenience thatthe long axis of the molecules align along the electric field (shown inFIG. 2) never occurs.

Upon applying a voltage, the incorporated dye starts emitting lightabove a certain operating voltage. Since the system possesses a negativedielectric anisotropy, the molecular alignment regulated by the electricfield agrees well with the molecular orientation in which the injectedcharges transport smoothly and such an inconvenience that the liquidcrystalline molecules stand perpendicular against the substrate as shownin FIG. 2 never occurs. Because of the alignment treatment, when the dyeis a dichroic dye, its transition moments lie along the liquid crystaldirector, and hence, the emitted light is polarized.

Here, only sandwiching electrodes are explained. Not explained arein-plane switching electrodes, which are fabricated on the samesubstrate. However, according to the present invention, the net effectsare exactly the same for both cases. In the case of in-plane switchingelectrodes, it is desired that the initial liquid crystal alignment isperpendicular to the electric field, and the alignment treatmentdescribed with respect to invention form 2 is preferred. Both electrodescan be opaque in this in-plane switching mode, and the photo-alignmenttechnique is preferred, since the metal electrodes are mechanicallyweak.

Invention Form 3

Another form of this invention is explained in FIG. 4. The differencebetween invention form 1 and 3 is that the liquid crystalline mixtureincludes a polymerizable compound. As polymerizable compound, everycompound is suitable as long as it is compatible with the liquidcrystalline compounds and polymerizes by heat or light treatment. Aphoto-polymerizable compound is preferred from the point of view ofprocess simplicity. A compound with a mesogenic group is preferred fromthe point of view of compatibility with the liquid crystalline compoundsand the alignment ability. A compound with more than two functionalgroups per molecule is preferred from the point of view of stabilizingthe orientation.

FIG. 4 shows a cross section of a luminescent device illustrating theconcept of invention form 3.

Explanation of the symbols:

401 compound with a negative dielectric anisotropy

402 fluorescent dye

403 polymerizable compounds

404 the first transparent substrate

405 transparent electrode

406 electrode

407 the second substrate

Examples of photo-polymerizable compounds are, as described in SPIE,Vol. 1455, p. 110, mixtures of thiolene and mercaptan (commercial name:NOA65), vinyl cinnamate derivatives or acrylate derivatives. Liquidcrystalline materials with photo-sensitive groups which are described inLiquid Crystals, Vol. 18, p. 319 (1995) also give preferred effects. Notonly low molecular weight compounds but also oligomers with functionalgroups are applicable.

In the same way as in invention form 1, a transparent electrode 405,such as ITO, is formed on the first transparent substrate 404 on which afew hundred-nm high pillar spacers (not shown in FIG. 4) are formedscores of μm apart from one after another using photo-lithographytechnique. The first transparent substrate 404 and the second substrate407, on which a counter electrode 406 is formed using, besides ITO, asmall work function metal, for example Li—Al (Li 0.2%) alloy, arealigned and fixed using an adhesive agent which is hardened either byheat or UV light. The fabricated cell is filled with a liquidcrystalline mixture which contains at least one compound with a negativedielectric anisotropy and a polymerizable compound and having optionallyincorporated a dye material and the filling port is fixed with UVhardening resin. The liquid crystalline compounds take a planeralignment. The cell can be heated above the isotropic temperature of theliquid crystalline compounds and cooled down slowly to make this planeralignment better, if necessary. The polymerization reaction or thecross-linking reaction is conducted using UV irradiation. A small amountof a photo-initiator may be added to the liquid crystalline compounds inorder to facilitate the polymerization or cross-linking reaction. Thisprocess can be either or both liquid crystal orientation stabilizationand/or pixel wall formation depending on whether a photo-mask is used ornot. It also depends on the amount of polymerizable compound. Uponapplying a voltage, the incorporated dye starts emitting light above acertain operating voltage. Since the system possesses a negativedielectric anisotropy, the molecular alignment regulated by the electricfield agrees well with the molecular orientation in which the injectedcharges transport smoothly and such an inconvenience that the liquidcrystalline molecules stand perpendicular to the substrate as shown inFIG. 2 never occurs.

The polymerization and/or the cross-linking improves the mechanicalstrength and it provides the advantage that the cell becomes quiteresistible against bending, which in fact is particularly suitable forplastic substrates.

Here, only sandwiching electrodes are explained. Not explained arein-plane switching electrodes, which are fabricated on the samesubstrate. However, according to the present invention, the net effectsare exactly the same for both cases.

Invention Form 4

Another form of this invention is explained in FIG. 5. The differencebetween invention form 3 and 4 is that at least one substrate is subjectto an alignment treatment.

FIG. 5 shows a cross section of a luminescent device illustrating theconcept of invention form 4.

Explanation of the symbols:

501 compound with a negative dielectric anisotropy

502 fluorescent dye

503 polymerizable compounds

504 the first transparent substrate

505 transparent electrode

506 electrode

507 the second substrate

508 alignment layer

509 alignment layer

In the same way as in invention form 2, a transparent electrode 505,such as ITO, is formed on the first transparent substrate 504 on which afew hundred-nm high pillar spacers (not shown in the FIG. 5) are formedscores of μm apart from one after another using photo-lithographytechnique. The electrode surface is rubbed using a cloth in order tocontrol the orientation of the liquid crystals. An alignment layer 508,composed of polyimide etc., can be coated on the first transparentsubstrate 504 and be rubbed after baking. For the alignment layer 508,besides polyimide, also a mixture of poly(styrene-sulfonate) andpoly(2,3-dihydrothieno[3,4-b]-1,4-dioxin) PEDOT) can be applied in orderto facilitate the hole injection from the transparent electrode 505. Thesurface of the second substrate 507, on which, besides ITO, a small workfunction metal electrode 506 is formed, is not necessarily rubbed, butit may be rubbed to make the liquid crystalline alignment stronger.Photo-alignment technique is preferably applied for aligning liquidcrystals on the second substrate 507, especially, when theinconvenience, as electrode peeling or surface scratching, occurs, dueto a mechanical treatment, like rubbing on a coated alignment layer 509like polyimide, since metal is generally soft and a small work functionmetal is particularly weak against mechanical treatment. Photo-alignmenttechnique is also applicable to the first transparent substrate 504.

The first transparent substrate 504 and the second substrate 507, atleast one of both is treated for liquid crystal alignment, are alignedand fixed using an adhesive agent which is hardened either by heat or UVlight. In the same way as in invention form 3, a liquid crystallinemixture that contains at least one compound with a negative dielectricanisotropy and having optionally incorporated a dye material, isintroduced into the fabricated cell and the filling port is fixed with aUV hardening resin. The liquid crystalline compounds align along thealignment treatment direction. The cell may be heated above theisotropic temperature of the liquid crystalline compounds and cooleddown slowly in order to get a better alignment. This process eliminatesthe problem, such as flow alignment. Applying a voltage during thecooling process, if necessary, further improves the liquid crystalalignment. Here, according to the present invention, since the systempossesses a negative dielectric anisotropy, long axis of the moleculesalign parallel to the substrate surface, and such an inconvenience thatthe long axis of the molecules align along the electric field (shown inFIG. 2) never occurs. After achieving the necessary alignment of theliquid crystalline compounds, in the same way as in invention form 3,the polymerization reaction or the cross-linking reaction is conductedusing UV irradiation. A small amount of a photo-initiator may be addedto the liquid crystalline compounds in order to facilitate thepolymerization or cross-linking reaction. As in invention form 3, thisprocess can be either or both liquid crystal orientation stabilizationand/or pixel wall formation depending on whether a photo-mask is used ornot. It also depends on the amount of polymerizable compound.

Upon applying a voltage, the incorporated dye starts emitting lightabove a certain operating voltage. Since the system possesses a negativedielectric anisotropy, the molecular alignment regulated by the electricfield agrees well with the molecular orientation in which the injectedcharges transport smoothly and such an inconvenience that the liquidcrystalline molecules stand perpendicular against the substrate as shownin FIG. 2 never occurs. Because of the alignment treatment, when the dyeis a dichroic dye, its transition moments lie along the liquid crystaldirector, and hence, the emitted light is polarized.

The polymerization and/or the cross-linking improves the mechanicalstrength and it provides the advantage that the cell becomes quiteresistible against bending, which in fact is particularly suitable forplastic substrates.

For the system where a polymerizable compound is used and the initialorientation of the liquid crystalline compounds is fixed, as ininvention forms 3 and 4, the inconvenient state that the long axis ofthe molecules align along the electric field is prevented to occur ifthe polymerization degree and/or the cross-linking density is highenough. However, to obtain a high polymerization degree and/orcross-linking density, a high content of a polymerizable compound isrequired. According to the present invention, since the inconvenientstate that the long axis of the molecules align along the electric fieldnever occurs, the content of compounds without polymerizable functionalgroup can be increased and a greater variety of compounds can beutilized. The process window becomes wider, too, since controlling thedegree of polymerization and/or cross-linking density is not so strict.If the content of the compound with a negative dielectric anisotropy isnot high enough as to give the whole system a negative dielectricanisotropy, long axis of molecules align along the electric field.According to the present invention, even in that case, owing to thecompound with a negative dielectric anisotropy, the threshold voltagefor aligning long axis of the molecules along the electric field becomeshigher and the inconvenient state in which long axis of the moleculesalign along the electric field is practically suppressed. Thus, in thiscase, too, the advantages, that a greater variety of compounds can beutilized and the process window becomes wider, exist.

Here, only sandwiching electrodes are explained. Not explained arein-plane switching electrodes, which are fabricated on the samesubstrate. However, according to the present invention, the net effectsare exactly the same for both cases. In the case of in-plane switchingelectrodes, it is desired that the initial liquid crystal alignment isperpendicular to the electric field, and the alignment treatmentdescribed with respect to invention form 2 is preferred. Both electrodescan be opaque in this in-plane switching mode, and the photo-alignmenttechnique is preferred, since the metal electrodes are mechanicallyweak.

Invention Form 5

Another form of this invention is explained in FIG. 6. The differencebetween invention form 3 and 5 is that after the polymerization and/orthe cross-linking reaction, the second substrate is removed and theelectrode is formed on the polymerized and/or cross-linked liquidcrystalline compounds.

FIG. 6 shows a cross section of a luminescent device illustrating theconcept of invention forms 5 and 7.

Explanation of the symbols:

601 compound with a negative dielectric anisotropy

602 fluorescent dye

603 polymerizable compounds

604 the first transparent substrate

605 transparent electrode

606 electrode

607 the second substrate

610 sealing layer

611 wall

In the same way as in invention form 3, a transparent electrode 605,such as ITO, is formed on the first transparent substrate 604 on which afew hundred-nm high pillar spacers (not shown in the FIG. 6) are formedscores of μm apart from one after another using photo-lithographytechnique. At the same time, walls 611 are formed instead of a sealingagent but in the same pattern as the sealing agent in invention form 3.The first transparent substrate 604 and the second substrate 607 onwhich the electrode is formed, if necessary, are aligned and fixed usinga pressure device. The cell is filled with a liquid crystalline mixturethat contains at least one compound with a negative dielectricanisotropy and a polymerizable compound having optionally incorporated adye material. The liquid crystalline compounds take a planer alignment.The cell can be heated above the isotropic temperature of the liquidcrystalline compounds and cooled down slowly to make this planeralignment better, if necessary. Applying a voltage during the coolingprocess, if necessary, further improves the liquid crystalline planeralignment. Here, according to the present invention, since the systempossesses a negative dielectric anisotropy, the long axis of themolecules align parallel to the substrate surface, and such aninconvenience that the long axis of the molecules align along theelectric field (shown in FIG. 2) never occurs. If a proper alignment isachieved without applying a voltage, the electrode on the secondsubstrate is not necessary.

The polymerization and/or the cross-linking reaction is conducted usingUV irradiation. A small amount of a photo-initiator may be added to theliquid crystalline compounds in order to facilitate the polymerizationand/or cross-linking reaction. As in invention form 3, this process canbe either or both liquid crystal orientation stabilization and/or pixelwall formation depending on whether a photo-mask is used or not. It alsodepends on the amount of polymerizable compound. After the reaction, thesecond substrate is removed. A small work function metal is formed in adesired pattern through a proper mask as the second electrode 606 on thepolymerized and/or cross-linked liquid crystalline compounds. If thismetal is active, the whole device is sealed after the second electrode606 formation without exposing the device to the air. This sealing layer610 is formed by subsequently depositing an inactive substance on theelectrode 606 and/or coating with epoxy resin.

The second substrate surface may be modified with a fluorinated polymeror a coupling agent to achieve a small surface energy for easy removalof the second substrate.

Upon applying a voltage, the incorporated dye starts emitting lightabove a certain operating voltage. Since the system possesses a negativedielectric anisotropy, the molecular alignment regulated by the electricfield agrees well with the molecular orientation in which the injectedcharges transport smoothly and such an inconvenience that the liquidcrystalline molecules stand perpendicular against the substrate as shownin FIG. 2 never occurs. The polymerization and/or the cross-linkingimproves the mechanical strength and it provides the advantage that thecell becomes quite resistible against bending, which in fact isparticularly suitable for plastic substrates, as in invention form 3. Ininvention form 5, the counter electrode can be formed on the liquidcrystalline compounds, and it is relatively easier, compared withinvention forms 1 to 4, to seal the whole device after the electrodeformation without exposing to the air. This enables to use a quite smallwork function metal as an electrode, which in fact leads to theadvantage that devices with a high charge injection efficiency, in otherwords, a high light emission efficiency are easily processed.

Invention Form 6

Another form of this invention is explained in FIG. 7. The differencebetween invention form 5 and 6 is that at least one substrate is subjectto an alignment treatment.

FIG. 7 shows a cross section of a luminescent device illustrating theconcept of invention form 6 and 8.

Explanation of the symbols:

701 compound with a negative dielectric anisotropy

702 fluorescent dye

703 polymerizable compounds

704 the first transparent substrate

705 transparent electrode

706 electrode

708 alignment layer

710 sealing layer

711 wall

In the same way as in invention form 2, a transparent electrode 705,such as ITO, is formed on the first transparent substrate 704 on which afew hundred-nm high pillar spacers (not shown in FIG. 7) are formedscores of μm apart from one after another using photo-lithographytechnique. The electrode surface is rubbed using a cloth in order tocontrol the orientation of the liquid crystals. An alignment layer 708,composed of polyimide etc., can be coated on the first transparentsubstrate 704 and be rubbed after baking in order to achieve a betteralignment of the liquid crystalline compounds. For the alignment layer708, besides polyimide, also a mixture of poly(styrene-sulfonate) andpoly(2,3-dihydrothieno[3,4-b]-1,4-dioxin) PEDOT) can be applied in orderto facilitate the hole injection from the transparent electrode 705. Inthe same way as in invention form 5, walls 711 are formed at the deviceboundaries when the spacers are formed. They also can be formed afterintroducing the liquid crystalline mixture through photo-polymerizationof the polymerizable compound incorporated in the liquid crystallinemixture using a photo-mask. The first transparent substrate 704 and thesecond substrate on which the electrode is formed, if necessary, arealigned and fixed using a pressure device. The cell is filled with aliquid crystalline mixture that contains at least one compound with anegative dielectric anisotropy and a polymerizable compound havingoptionally incorporated a dye material. The liquid crystalline compoundsalign along the rubbing direction. The cell can be heated above theisotropic temperature of the liquid crystalline compounds and cooleddown slowly to make this alignment better, if necessary. This processeliminates the problem, such as flow alignment. Applying a voltageduring the cooling process, if necessary, further improves the liquidcrystal planer alignment.

Here, according to the present invention, since the system possesses anegative dielectric anisotropy, the long axis of the molecules alignparallel to the substrate surface, and such an inconvenience that thelong axis of the molecules align along the electric field (shown in FIG.2) never occurs. If a proper alignment is achieved only with thealignment treatment of the first substrate, the electrode or alignmentlayer on the second substrate and voltage applying process are notnecessary. After achieving the necessary alignment of the liquidcrystalline compounds, in the same way as in invention form 3, thepolymerization reaction or the cross-linking reaction is conducted usingUV irradiation. A small amount of a photo-initiator may be added to theliquid crystalline compounds in order to facilitate the polymerizationor cross-linking reaction. This process can be either or both liquidcrystal orientation stabilization and/or pixel wall formation dependingon whether a photo-mask is used or not. It also depends on the amount ofpolymerizable compound. In the same way as in invention form 5, thesecond substrate is removed after the polymerization and/orcross-linking reaction. A small work function metal is formed in adesired pattern through a proper mask as the second electrode 706 on thepolymerized and/or cross-linked liquid crystalline compounds. If thismetal is active, the whole device is sealed after the second electrode706 formation without exposing the device to the air. This sealing layer710 is formed by subsequently depositing an inactive substance on theelectrode 706 and/or coating with epoxy resin.

The second substrate surface may be coated with a fluorinated alignmentlayer or modified with PTFE as described by Wittman et al. in Nature,Vol. 352, p. 414 (1991) for a good alignment and an easy removal of thesecond substrate.

Upon applying a voltage, the incorporated dye starts emitting lightabove a certain operating voltage, as in invention form 5. Since thesystem possesses a negative dielectric anisotropy, the molecularalignment regulated by the electric field agrees well with the molecularorientation in which the injected charges transport smoothly and such aninconvenience that the liquid crystalline molecules stand perpendicularagainst the substrate as shown in FIG. 2 never occurs. Thepolymerization and/or the cross-linking improves the mechanical strengthand it provides the advantage that the cell becomes quite resistibleagainst bending, which in fact is particularly suitable for plasticsubstrates as in invention form 3. In invention form 6, the counterelectrode can be formed on the liquid crystalline compounds, and it isrelatively easier, compared with invention forms 1 to 4, to seal thewhole device after the electrode formation without exposing to the air.This enables to use quite a small work function metal as an electrode,which in fact leads to the advantage that devices with a high chargeinjection efficiency, in other words, a high light emission efficiencyare easily processed. In invention form 6, since the liquid crystallinecompounds align uniaxialy, when the dye is a dichroic dye, the emittedlight is polarized.

Invention Form 7

Another form of this invention is explained in FIG. 6. The differencebetween invention form 5 and 7 is the method to fabricate thepolymerized and/or the cross-linked liquid crystalline compounds.

In the same way as in invention form 1, a transparent electrode 605,such as ITO, is formed on the first transparent substrate 604. As ininvention form 5 a few hundred-nm high pillar spacers (not shown in theFIG. 6) and walls 611 may be formed using photo-lithography technique,if necessary. A film is formed on the electrode 605 which is composed ofa liquid crystalline mixture that contains at least one compound with anegative dielectric anisotropy and a polymerizable compound havingoptionally incorporated a dye material dissolved in a suitable organicsolvent. The film is coated from the solution using spin-coating,printing, dipping or blade coating technique. After baking the substrateand vapourising the solvent, the liquid crystalline compounds take aplaner alignment. Here, incorporating a small amount of a detergentfacilitates the planer alignment. The polymerization and/or thecross-linking reaction is conducted using UV irradiation. A small amountof a photo-initiator may be added to the liquid crystalline mixture tofacilitate the polymerization and/or the cross-linking reaction.

As detergents, fluorinated detergents, for example, FC-171 (Commercialname: 3M), are particularly effective. Detergents that havephoto-reactive functional groups, for example, FX-13 (Commercial name:3M) are also preferred. The detergent is incorporated into thepolymerizable agent in an amount of from 0.6 to 1% by weight.

The film thickness can be controlled by the concentration of thesolution, the spin speed for spin-coating, the dipping speed fordipping, and the gap between the substrates and the blade for printingand blade coating technique. A film thickness of 50 nm to 1 μm ispreferred from the point of view of lowering the operation voltage, anda film thickness of 100 nm to 300 nm is particularly preferred.

In the same way as in invention form 5, a small work function metal isformed in a desired pattern through a proper mask as the secondelectrode 606 on the polymerized and/or cross-linked liquid crystallinecompounds. If this metal is active, the whole device is sealed after theformation of the second electrode 606 without exposing the device to theair. This sealing layer 610 is formed by subsequently depositing aninactive substance on the electrode 606 and/or coating with epoxy resin.

Upon applying a voltage, the incorporated dye starts emitting lightabove a certain operating voltage. Since the system possesses a negativedielectric anisotropy, the molecular alignment regulated by the electricfield agrees well with the molecular orientation in which the injectedcharges transport smoothly and such an inconvenience that the liquidcrystalline molecules stand perpendicular against the substrate as shownin FIG. 2 never occurs. The polymerization and/or the cross-linkingimproves the mechanical strength and it provides the advantage that thecell becomes quite resistible against bending, which in fact isparticularly suitable for plastic substrates, as in invention form 3. Ininvention form 7, the counter electrode can be formed on the liquidcrystalline compounds, and it is relatively easier, compared withinvention forms 1 to 4, to seal the whole device after the electrodeformation without exposing to the air. This enables to use a quite smallwork function metal as an electrode, which in fact leads to theadvantage that devices with a high charge injection efficiency, in otherwords, a high light emission efficiency are easily processed.

Here only the process, in which the pixel walls are formed afterintroducing the liquid crystalline mixture into the cell, is described.It is also possible to form the pixel walls using photo-lithographytechnique on the first substrate at the same time of forming pillarspacers, and fill each pixel with the liquid crystalline mixturecontaining different color material, if necessary, using e.g. an ink-jetprinting technique.

Invention Form 8

Another form of this invention is explained in FIG. 7. The differencebetween invention form 7 and 8 is that at least one substrate is subjectto an alignment treatment.

In the same way as in invention form 2, a transparent electrode 705,such as ITO, is formed on the first transparent substrate 704. As ininvention form 6, a few hundred-nm high pillar spacers (not shown inFIG. 7) may be formed scores of μm apart from one after another usingphoto-lithography technique, if necessary. Also walls 711 may be formedat the device boundaries when the spacers are formed. They also can beformed after introducing the liquid crystalline mixture throughphoto-polymerization of the polymerizable compound incorporated in theliquid crystalline mixture using a photo-mask, if necessary. Theelectrode surface is rubbed using a cloth in order to control theorientation of the liquid crystals. An alignment layer 708, composed ofpolyimide etc., can be coated on the first transparent substrate 704 andbe rubbed after baking in order to achieve a better alignment of theliquid crystalline compounds. For the alignment layer 708, besidespolyimide, also a mixture of poly(styrene-sulfonate) andpoly(2,3-dihydrothieno[3,4-b]-1,4-dioxin) PEDOT) can be applied in orderto facilitate the hole injection from the transparent electrode 705. Analignment layer 708 composed of a fluorinated polymer or PTFE, asdescribed in Nature, Vol. 352, p. 414 (1991) by Wittman et al., may beused to obtain a better alignment.

In the same way as in invention form 7, a film is formed on the firstsubstrate subjected to an alignment treatment which is composed of aliquid crystalline mixture that contains at least one compound with anegative dielectric anisotropy and a polymerizable compound havingoptionally incorporated a dye material dissolved in a suitable organicsolvent. The film is coated from the solution using spin-coating,printing, dipping or blade coating technique. After baking the substrateand vapourising the solvent, the liquid crystalline compounds take aplaner alignment. Here, incorporating a small amount of a detergentfacilitates the planer alignment. The polymerization and/or thecross-linking reaction is conducted using UV irradiation. A small amountof a photo-initiator may be added to the liquid crystalline mixture tofacilitate the polymerization and/or the cross-linking reaction. As ininvention form 6, this process can be either or both liquid crystalorientation stabilization and/or pixel wall formation depending onwhether a photo-mask is used or not. It also depends on the amount ofpolymerizable compound. Since the first substrate has an alignmenttreatment, the liquid crystalline compounds align along the alignmenttreatment direction. This alignment is fixed by the polymerizationand/or the cross-linking reaction.

In the same way as in invention form 7, a small work function metal isformed in a desired pattern through a proper mask as the secondelectrode 706 on the polymerized and/or cross-linked liquid crystallinecompounds. If this metal is active, the whole device is sealed after theformation of the second electrode 706 without exposing the device to theair. This sealing layer 710 is formed by subsequently depositing aninactive substance on the electrode 706 and/or coating with epoxy resin.

In the similar way as in invention form 7, upon applying a voltage, theincorporated dye starts emitting light above a certain operatingvoltage. Since the system possesses a negative dielectric anisotropy,the molecular alignment regulated by the electric field agrees well withthe molecular orientation in which the injected charges transportsmoothly and such an inconvenience that the liquid crystalline moleculesstand perpendicular against the substrate as shown in FIG. 2 neveroccurs. Since the orientation of the liquid crystalline compounds isfixed through the polymerization and/or the cross-linking reaction, anionic transport that was observed rarely if ever at extremely highvoltage applications is suppressed. The polymerization and/or thecross-linking improves the mechanical strength and it provides theadvantage that the cell becomes quite resistible against bending, whichin fact is particularly suitable for plastic substrates, as in inventionform 3. In invention form 8, the counter electrode can be formed on theliquid crystalline compounds, and it is relatively easier, compared withinvention forms 1 to 4, to seal the whole device after the formation ofthe electrode without exposing to the air. This enables to use a quitesmall work function metal as an electrode, which in fact leads to theadvantage that devices with a high charge injection efficiency, in otherwords, a high light emission efficiency are easily processed. Ininvention form 8, since the liquid crystalline compounds have anuniaxial alignment, when the incorporated dye is dichroic, the emittedlight is polarized.

As in invention form 7, it is also possible to form the pixel wallsusing photo-lithography technique on the first substrate at the sametime of forming pillar spacers, and fill each pixel with the liquidcrystalline mixture containing different color material, if necessary,using e.g. an ink-jet printing technique.

Invention Form 9

Another form of this invention is explained in FIG. 8. The differencebetween invention form 7 and 9 is that the layer of the liquidcrystalline compounds is divided into two layers, a hole transport layerand an electron transport layer.

FIG. 8 shows a cross section of a luminescent device illustrating theconcept of invention form 9.

Explanation of the symbols:

801 compound with a negative dielectric anisotropy

802 fluorescent dye or hole transport facilitating compound

803 polymerizable compounds

804 the first transparent substrate

805 transparent electrode

806 electrode

810 sealing layer

811 wall

812 the second fluorescent dye or electron transport facilitatingcompound

In the same way as in invention form 7, a transparent electrode 805,such as ITO, is formed on the first transparent substrate 804. As ininvention form 6, a few hundred-nm high pillar spacers (not shown inFIG. 8) may be formed scores of μm apart from one after another usingphoto-lithography technique, if necessary. Also walls 811 may be formedat the device boundaries when the spacers are formed. They also can beformed after introducing the liquid crystalline mixture throughphoto-polymerization of the polymerizable compound incorporated in theliquid crystalline mixture using a photo-mask, if necessary. A film isformed on the electrode 805 which is composed of a liquid crystallinemixture that contains at least one compound with a negative dielectricanisotropy and a polymerizable compound having optionally incorporated amaterial that facilitates the hole transport, for example, aminederivatives, represented by structures III(a), III(b) and III(c),dissolved in a suitable organic solvent.

The film is coated from the solution using spin-coating, printing,dipping or blade coating technique. After baking the substrate andvapourising the solvent, the liquid crystalline compounds take a planeralignment. Here, incorporating a small amount of a detergent facilitatesthe planer alignment. The polymerization and/or the cross-linkingreaction is conducted using UV irradiation. A small amount of aphoto-initiator may be added to the liquid crystalline mixture tofacilitate the polymerization and/or the cross-linking reaction. Thisprocess can be mainly liquid crystal orientation stabilization but itcan also be pixel wall formation. A film thickness of 10 nm to 1 μm ispreferred from the point of view of lowering the operation voltage, anda film thickness of 20 nm to 300 nm is particularly preferred.

Another film is formed on the polymerized and/or the cross-linked liquidcrystalline compounds, which is composed of a liquid crystalline mixturethat contains at least one compound with a negative dielectricanisotropy and a polymerizable compound having optionally incorporated amaterial that facilitates the electron transport, for example, triazolederivatives, represented by structures III(d), III(e) and III(f),dissolved in a suitable organic solvent.

The film is coated from the solution using spin-coating, printing,dipping or blade coating technique. After baking the substrate andvapourising the solvent, the liquid crystalline compounds take a planeralignment. Here, incorporating a small amount of a detergent facilitatesthe planer alignment. The polymerization and/or the cross-linkingreaction is conducted using UV irradiation. A small amount of aphoto-initiator may be added to the liquid crystalline mixture tofacilitate the polymerization and/or the cross-linking reaction. Thisprocess can be mainly liquid crystal orientation stabilization but itcan also be pixel wall formation. A film thickness of 10 nm to 1 μm ispreferred from the point of view of lowering the operation voltage, anda film thickness of 20 nm to 300 nm is particularly preferred. Thematerials which facilitate the electron transport are often fluorescent.Fluorescent dyes with a desired wavelength region may be incorporated ineither layer or both layers, if necessary. If the liquid crystallinecompounds themselves facilitate the hole and the electron transport, thesecond layer is not necessary and only incorporating a dye is enough.

As the second layer, an amorphous evaporated 10 nm to 1 μm-thick film ofthe material that facilitates the electron transport may be used ifnecessary. On the two layered polymerized and/or cross-linked liquidcrystalline film, in the same way as in invention form 5, a small workfunction metal is formed in a desired pattern through a proper mask asan electron injecting electrode. If this metal is active, the wholedevice is sealed after the formation of the second electrode 806 withoutexposing the device to the air. The sealing layer 810 is formed bysubsequently depositing an inactive substance on the electrode 806and/or coating with epoxy resin.

Upon applying a voltage, the incorporated dye starts emitting lightabove a certain operating voltage. Since the system possesses a negativedielectric anisotropy, the molecular alignment regulated by the electricfield agrees well with the molecular orientation in which the injectedcharges transport smoothly and such an inconvenience that the liquidcrystalline molecules stand perpendicular against the substrate as shownin FIG. 2 never occurs. The polymerization and/or the cross-linkingimproves the mechanical strength and it provides the advantage that thecell becomes quite resistible against bending, which in fact isparticularly suitable for plastic substrates, as in invention form 3. Ininvention form 9, the counter electrode can be formed on the liquidcrystalline compounds, and it is relatively easier, compared withinvention forms 1 to 4, to seal the whole device after the formation ofthe electrode without exposing to the air. This enables to use a quitesmall work function metal as an electrode, which in fact leads to theadvantage that devices with a high charge injection efficiency, in otherwords, a high light emission efficiency are easily processed. Ininvention form 9, since the layer for the hole transport and the layerfor the electron transport are separate, a preferred combination ofmaterials can be designed which in fact leads to the advantage thatdevices with a high charge transport efficiency, in other words, a highlight emission efficiency are easily processed.

Invention Form 10

Another form of this invention is explained in FIG. 9. The differencebetween invention form 9 and 10 is that at least one substrate issubject to an alignment treatment.

FIG. 9 shows a cross section of a luminescent device illustrating theconcept of invention form 10.

Explanation of the symbols:

901 compound with a negative dielectric anisotropy

902 fluorescent dye or hole transport facilitating compound

903 polymerizable compounds

904 the first transparent substrate

905 transparent electrode

906 electrode

908 alignment layer

910 sealing layer

911 wall

912 the second fluorescent dye or electron transport facilitatingcompound

In the same way as in invention form 2, a transparent electrode 905,such as ITO, is formed on the first transparent substrate 904. As ininvention form 6, a few hundred-nm high pillar spacers (not shown inFIG. 8) may be formed scores of μm apart from one after another usingphoto-lithography technique, if necessary. Also walls 911 may be formedat the device boundaries when the spacers are formed. They also can beformed after introducing the liquid crystalline mixture throughphoto-polymerization of the polymerizable compound incorporated in theliquid crystalline mixture using a photo-mask, if necessary. Theelectrode surface is rubbed using a cloth in order to control theorientation of the liquid crystals. An alignment layer 908, composed ofpolyimide etc., can be coated on the first transparent substrate 904 andbe rubbed after baking in order to achieve a better alignment of theliquid crystalline compounds. For the alignment layer 908, besidespolyimide, also a mixture of poly(styrene-sulfonate) andpoly(2,3-dihydrothieno[3,4-b]-1,4-dioxin) PEDOT) can be applied in orderto facilitate the hole injection from the transparent electrode. Afluorinated polymer or PTFE film, as described in Nature, Vol. 352, p.414 (1991) by Wittman et al., can also be used as an alignment layer908. A mixture of poly(styrene-sulfonate) andpoly(2,3-dihydrothieno[3,4-b]-1,4-dioxin) PEDOT) on the rubbedfluorinated polymer or PTFE film can also be used as an alignment layer908.

In the same way as in invention form 9, a film is formed from a solutionwhich is composed of a liquid crystalline mixture that contains at leastone compound with a negative dielectric anisotropy and a polymerizablecompound having optionally incorporated a material, that facilitates thehole transport, dissolved in a suitable organic solvent. The film iscoated from the solution using spin-coating, printing, dipping or bladecoating technique. After baking the substrate and vapourising thesolvent, the liquid crystalline compounds take a planer alignment. Here,incorporating a small amount of a detergent facilitates the planeralignment. The polymerization and/or the cross-linking reaction isconducted using UV irradiation. A small amount of a photo-initiator maybe added to the liquid crystalline mixture to facilitate thepolymerization and/or the cross-linking reaction. This process can bemainly liquid crystal orientation stabilization but it can also be pixelwall formation. A film thickness of 10 nm to 1 μm is preferred from thepoint of view of lowering the operation voltage, and a film thickness of20 nm to 300 nm is particularly preferred. Since the first substrate issubject to an alignment treatment, the liquid crystalline compoundsalign along the alignment direction, and the orientation is fixed by thepolymerization and/or the cross-linking reaction. Polarized UV light canbe used in order to facilitate the liquid crystalline alignment duringthe polymerization and/or the cross-linking reaction. Polarized UV lightexcites only the molecules whose transition moment lies along thepolarization direction selectively. Consequently, the director of theliquid crystalline compounds aligns parallel or perpendicular to thepolarization direction depending on the nature of the photo-sensitivegroup. If the polarized UV irradiation is sufficient to align the liquidcrystalline compounds, the first substrate alignment treatment can beomitted.

Another film is formed on the polymerized and/or the cross-linked liquidcrystalline compounds, which is composed of a liquid crystalline mixturethat contains at least one compound with a negative dielectricanisotropy and a polymerizable compound having optionally incorporated amaterial, that facilitates the electron transport, dissolved in asuitable organic solvent. The film is coated from the solution usingspin-coating, printing, dipping or blade coating technique. After bakingthe substrate and vapourising the solvent, the liquid crystallinecompounds take a planer alignment. Since the underneath layer isaligned, the coated liquid crystalline compounds also align. Thepolymerization and/or the cross-linking reaction is conducted using UVirradiation. The orientation of the liquid crystalline compounds isfixed. Polarized UV light can be used in order to facilitate the liquidcrystalline alignment during the polymerization and/or the cross-linkingreaction, as for the first layer of the liquid crystalline compounds.This process can be mainly liquid crystal orientation stabilization butit can also be pixel wall formation. A film thickness of 10 nm to 1 μmis preferred from the point of view of lowering the operation voltage,and a film thickness of 20 nm to 300 nm is particularly preferred. Thematerials which facilitate the electron transport are often fluorescent.Fluorescent dyes with a desired wavelength region may be incorporated ineither layer or both layers, if necessary. If the liquid crystallinecompounds themselves facilitate the hole and the electron transport, thesecond layer is not necessary and only incorporating a dye is enough.

On the two layered polymerized and/or cross-linked liquid crystallinefilm, in the same way as in invention form 5, a small work functionmetal is formed in a desired pattern through a proper mask as anelectron injecting electrode. If this metal is active, the whole deviceis sealed after the formation of the second electrode 906 withoutexposing the device to the air. The sealing layer 910 is formed bysubsequently depositing an inactive substance on the electrode 906and/or coating with epoxy resin.

Upon applying a voltage, the incorporated dye starts emitting lightabove a certain operating voltage. Since the system possesses a negativedielectric anisotropy, the molecular alignment regulated by the electricfield agrees well with the molecular orientation in which the injectedcharges transport smoothly and such an inconvenience that the liquidcrystalline molecules stand perpendicular against the substrate as shownin FIG. 2 never occurs. The polymerization and/or the cross-linkingimproves the mechanical strength and it provides the advantage that thecell becomes quite resistible against bending, which in fact isparticularly suitable for plastic substrates, as in invention form 3. Ininvention form 10, the counter electrode can be formed on the liquidcrystalline compounds, and it is relatively easier, compared withinvention forms 1 to 4, to seal the whole device after the formation ofthe electrode without exposing to the air. This enables to use a quitesmall work function metal as an electrode, which in fact leads to theadvantage that devices with a high charge injection efficiency, in otherwords, a high light emission efficiency are easily processed. Ininvention form 10, since the layer for the hole transport and the layerfor the electron transport are separate, a preferred combination ofmaterials can be designed which in fact leads to the advantage thatdevices with a high charge transport efficiency, in other words, a highlight emission efficiency are easily processed. In invention form 10,since the liquid crystalline compounds have an uniaxial alignment, whenthe incorporated dye is dichroic, the emitted light is polarized.

In this invention form, only the method in which both layers arepolymerized and/or cross-linked films is explained, however, accordingto the present invention, the method is not limited to this embodiment.Combinations of the above mentioned methods are also effective, forexample, an embodiment where the first layer is a polymerized and/or across-linked film and a cell is fabricated in such a way that the secondlayer can be formed of liquid crystalline compounds without apolymerization and/or a cross-linking reaction and/or evaporation.

In this invention form, only the instances for two layers are explained,however, according to the present invention, the number of layers is notlimited to this embodiment. Any preferred number of layers can befabricated by repeating the same process after the polymerization and/orthe cross-linking reaction. It is needless to say that the amount andthe kind of incorporated compounds for each layer can be adjusted toachieve the optimum properties.

The present invention is in the following explained in detail withworking examples.

EXAMPLE 1

A stripe electrode of ITO with 2 mm width is formed on a glass substrateusing the photo-lithography technique. Bead spacers with 1.6 μm indiameter are spread and an adhesive agent is painted. Two substrates arealigned so that the two electrodes face and cross each other to overlapas squares. The substrates are pressed and baked at 150° C. for 2 hours.

As liquid crystalline mixture that contains at least one compound with anegative dielectric anisotropy, the following mixture is used:

The dielectric anisotropy Δε of the liquid crystalline mixture is: −6.7.

The mixture together with 0.3% by weight Coumarin 6 is introduced intothe fabricated cell. Upon applying a D.C. 50 V to the electrodes a greenemission from Coumarin 6 is observed.

The orientation of the liquid crystalline compounds is checked using apolarized microscope and a Shrielen texture is observed. It is foundthat the long axis of the molecules lies parallel to the substrate.

Besides D.C. voltage, also A.C. voltage is applicable to the cell, sincethe cell has a symmetrical structure. Upon applying a 40V A.C. (10-100Hz) to the same cell, a green emission from Coumarin 6 is observed. Inthis case, it is also found that the long axis of the molecules liesparallel to the substrate.

COMPARATIVE EXAMPLE 1

Cyano-biphenyl liquid crystalline compounds (Commercial name: E7,Merck), whose dielectric anisotropy is positive, are, together with 0.3%by weight Coumarin 6, introduced into the cell fabricated in example 1.A D.C. voltage up to 150 V is applied to the electrodes but no lightemission is observed. Applying a higher voltage than 150 V brings aboutelectric breakdown.

The orientation of the liquid crystalline compounds is checked using apolarized microscope. A Shrielen texture is observed when no voltage isapplied, which in fact means that the long axis of the molecules liesparallel to the substrate. When a D.C. voltage higher than 15 V isapplied, black state is observed and it does not change when the cell isrotated. This means the long axis of the molecules stand perpendicularto the substrate.

EXAMPLE 2

In the same way as in example 1, a stripe electrode of ITO is formed ona glass substrate. A polyimide alignment layer (commercial name AL-3046,JSR Corporation) is spin-coated on the substrate and the substrate isbaked at 200° C. for 1 hour and is rubbed. After spreading bead spacers,as in example 1, cells are fabricated. Two substrates are aligned sothat the two electrodes face and cross each other to overlap as squaresand the rubbing directions are anti-parallel. In the same way as inexample 1, the mixture as disclosed in example 1 is used as liquidcrystalline mixture that contains at least one compound with a negativedielectric anisotropy, and this mixture together with 0.3% by weight ofCoumarin 6 is introduced into the fabricated cell. Upon applying a D.C.50 V to the electrodes a green emission from Coumarin 6 is observed. Thepolarization of the emitted light is checked using a polarizer, and itis found that the light is polarized.

The orientation of the liquid crystalline compounds is checked using apolarized microscope. It is found that the long axis of the moleculeslies parallel to the substrate and the director of the liquidcrystalline compounds aligns along the rubbing direction which directioncoincides with the polarization direction of the emitted light.

Since the cell structure is symmetrical, a green emission is observedwhen a ±50V AC voltage is applied regardless of the polarity.

EXAMPLE 3

In the same way as in example 2, cells are fabricated. As liquidcrystalline mixture that contains at least one compound with a negativedielectric anisotropy, the following mixture is used:

25% by weight

25% by weight

25% by weight

25% by weight

The mixture together with 0.3% by weight of Coumarin 6 is introducedinto the cell. Upon applying a D.C. 20 V to the electrodes a greenemission from Coumarin 6 is observed. It is found that the operatingvoltage is lowered significantly compared with example 1. Thepolarization of the emitted light is checked using a polarizer, and itis found that the light is polarized along the rubbing direction.

The ionization potential of the liquid crystalline mixture is measuredusing a Riken-keiki AC-2 photo-electron spectrometer and it is found tobe 6.03 eV. For comparison, the ionization potential of the mixture ofexample 1 is tried to measure. However, it is higher than 6.1 eV, themeasurable limit of AC-2, and therefore cannot be measured.

The orientation of the liquid crystalline compounds is checked using apolarized microscope. It is found that the long axis of the moleculeslies parallel to the substrate and the director of the liquidcrystalline compounds aligns along the rubbing direction.

EXAMPLE 4

In the same way as in example 1, a stripe electrode of ITO is formed ona glass substrate, i.e. the first substrate. Using a positive photoresist (commercial name TFR H, Tokyo Ohka-kogyo Co. Ltd.) and itsthinner, 300 nm high pillar spacers, which are 10 μm×10 μm square shapedand 200 μm apart from one after another, are formed and the substrate isbaked at 200° C. for 2 hours under flowing nitrogen condition to hardenthe novolak resin. A mixture of poly(styrenesulfonate) andpoly(2,3-dihydrothieno[3,4-b]-1,4-dioxin) (PEDOT) purchased from Aldrichis spin-coated on the substrate and the substrate is baked at 100° C.for 1 hour under flowing nitrogen condition and is rubbed. As the secondsubstrate, Li—Al (Li 0.2%) alloy is evaporated on a glass substrate anda stripe electrode with 2 mm width is formed using the photo-lithographytechnique. The second substrate is rubbed without an alignment layer.The first and the second substrates are aligned so that the twoelectrodes face and cross each other to overlap as squares and therubbing directions are anti-parallel. The two substrates are fixed witha UV curable agent.

As liquid crystalline mixture that contains at least one compound with anegative dielectric anisotropy, the mixture of example 3 is used. As adiacrylate having a mesogen moiety,bis{4-6-(1-oxo-2-propenyl)oxyhexyloxybenzoicacid}2-methyl-1,4′-phenyleneester, having the following structure:

is incorporated in an amount of 3% by weight and Coumarin 6 isincorporated in an amount of 0.3% by weight. The whole mixture isintroduced into the cell. The cell is heated up to 90° C. and cooleddown slowly. During the cooling period a 5 V A.C. voltage is applied.After having a uniform orientation, the diacrylate is polymerized and/orcross-linked by UV irradiation. Upon applying a 10 V D.C. voltage (ITO:anode, Li—Al (Li 0.2%) alloy: cathode), a green emission from Coumarin 6is observed. The polarization of the emitted light is checked using apolarizer, and it is found that the light is polarized along the rubbingdirection.

The orientation of the liquid crystalline compounds is checked using apolarized microscope. It is found that the long axis of the moleculeslies parallel to the substrate and the director of the liquidcrystalline compounds aligns along the rubbing direction.

EXAMPLE 5

In the same way as in example 4 cells are fabricated, except thefollowing three points: that is, no Li—Al (Li 0.2%) alloy electrode onthe second substrate, no sealing agent and no A.C. voltage applicationduring the cooling process.

As a liquid crystalline mixture that contains at least one compound witha negative dielectric anisotropy, the mixture of example 1 is used. Thediacrylate of example 4 having a mesogen moiety, is incorporated in anamount of 30% by weight and Coumarin 6 is incorporated in an amount of0.3% by weight. The whole mixture is introduced into the cell. Thefixing of the substrates is conducted in that using a pressure devicethe diacrylate is polymerized and/or cross-linked by UV irradiation. Thesecond substrate is removed and Li—Al (Li 0.2%) alloy is evaporated toobtain a desired pattern through a proper mask. After the evaporation,without exposing the sample to the air, SiO is evaporated. The sample issealed with a glass substrate and a UV curable resin under flowingnitrogen condition.

Upon applying a 15V D.C. voltage (ITO: anode, Li—Al (Li 0.2%) alloy:cathode), a green emission from Coumarin 6 is observed. The polarizationof the emitted light is checked using a polarizer, and it is found thatthe light is polarized along the rubbing direction.

EXAMPLE 6

In the same way as in example 1, an ITO electrode is formed on the firstglass substrate. A mixture of poly(styrenesulfonate) andpoly(2,3-dihydrothieno[3,4-b]-1,4-dioxin) (PEDOT) purchased from Aldrichis spin-coated on the substrate and the substrate is baked at 100° C.for 1 hour under flowing nitrogen condition and the substrate is rubbed.

The mixture of example 1, 30% by weight of the diacrylate of example 4having mesogen moiety, 0.3% by weight of a detergent (commercial nameFX-13, 3M) and 3% by weight of triphenyldiamine (structure III(b)) aremixed and dissolved in propylene-glycol-monomethyl-ether-acetate (PGMEA)in such a way that a 5% by weight solution is prepared. The mixture isspin-coated from this solution on the first substrate and the substrateis baked at 80° C. for 1 hour. The orientation of the liquid crystallinecompounds is checked using a polarized microscope and it is found thatthe liquid crystalline compounds align along the rubbing direction. Thediacrylate monomers are polymerized and/or cross-linked with a UVirradiation under flowing nitrogen condition. The film thickness is 400nm.

As the second layer, (8-hydroxyquinoline) aluminium (Purchased fromAldrich) is evaporated so that the film thickness is 60 nm.

In the same way as in example 5, Li—Al is coevaporated as a cathodeelectrode and without exposing the sample to the air, SiO is evaporated.The sample is sealed with a glass substrate and a UV curable resin underflowing nitrogen condition.

Upon applying a 18V D.C. voltage (ITO: anode, Li—Al (Li 0.2%) alloy:cathode), a green emission from (8-hydroxyquinoline) aluminium isobserved.

EXAMPLE 7

In the same way as in example 6, an ITO electrode is formed on the firstglass substrate. A mixture of poly(styrenesulfonate) andpoly(2,3-dihydrothieno[3,4-b]-1,4-dioxin) (PEDOT) purchased from Aldrichis spin-coated on the substrate and the substrate is baked at 100° C.for 1 hour under flowing nitrogen condition and the substrate is rubbed.

In the same way as in example 6, the mixture of example 1, 30% by weightof a diacrylate having a mesogen moiety and having the followingstructure,

0.3% by weight of a detergent (commercial name FX-13, 3M) and 0.3% byweight of Coumarin 6 are mixed and dissolved in chloroform (CHCl₃) insuch a way that a 5% by weight solution is prepared. The mixture isspin-coated on the first substrate and the substrate is baked at 80° C.for 1 hour. The substrate was heated up to 120° C. and cooled downslowly. The orientation of the liquid crystalline compounds is checkedusing a polarized microscope and it is found that the liquid crystallinecompounds align along the rubbing direction. The diacrylate monomers arepolymerized and/or cross-linked with UV irradiation under flowingnitrogen condition. The film thickness is 300 nm.

In the same way as in example 5, Li—Al (Li 0.2%) alloy is evaporated asa cathode electrode and without exposing the sample to the air, SiO isevaporated. The sample is sealed with a glass substrate and a UV curableresin under flowing nitrogen condition.

Upon applying a 17V D.C. voltage (ITO: anode, Li—Al (Li 0.2%) alloy:cathode), a green polarized light emission from Coumarin 6 is observed.

EXAMPLE 8

In the same way as in example 7 a polymerized and/or cross-linked liquidcrystalline sample cell is prepared. The only difference is that thediacrylate of example 7 having mesogen moiety is replaced by thediacrylate having the following structure

Upon applying a 16V D.C. voltage (ITO: anode, Li—Al (Li 0.2%) alloy:cathode), a green light emission is observed.

1. An electro-luminescent material, characterized in that it comprises aliquid crystalline mixture containing at least one liquid crystallinecompound with a negative dielectric anisotropy.
 2. Anelectro-luminescent material as claimed in claim 1, characterized inthat the liquid crystalline compound with a negative dielectricanisotropy is present in the electro-luminescent material in an amountsufficient that the whole electro-luminescent material shows adielectric anisotropy <_(—)0.
 3. An electro-luminescent material asclaimed in claim 1, characterized in that the liquid crystallinecompound with a negative dielectric anisotropy contains at least one ofthe following units a to f:

wherein Hal is fluorine, chlorine, bromine.
 4. An electro-luminescentmaterial as claimed in claim 3, characterized in that the liquidcrystalline compound with a negative dielectric anisotropy contains atleast one unit a.
 5. An electro-luminescent material as claimed in claim1, characterized in that it contains at least one compound whoseionization potential is lower than 6.1 eV.
 6. An electro-luminescentmaterial as claimed in claim 5, characterized in that the at least onecompound whose ionization potential is lower than 6.1 eV is a tolanederivative.
 7. An electro-luminescent material as claimed in claim 6,characterized in that the tolane derivative is a compound of thefollowing formula:

wherein n is an integer from 1 to 5 and R is an alkyl group whose carbonnumber is from 1 to 7 or an alkylcyclohexyl group whose carbon number ofthe alkyl group is from 1 to
 7. 8. An electro-luminescent material asclaimed in claim 1, characterized in that the liquid crystalline mixtureshows a nematic phase ac least at room temperature.
 9. Anelectro-luminescent material as claimed in claim 1, characterized inthat it contains at least one fluorescent dye.
 10. An electroluminescentmaterial as claimed in claim 9, characterized in that the fluorescentdye is a dichroic dye.
 11. An electro-luminescent device comprising afirst transparent substrate having a transparent electrode on its innersurface and a second substrate having an electrode on its inner surface,the space between both substrates is formed as a cell, characterized inthat the cell contains an electro-luminescent material as claimed inclaim
 1. 12. An electro-luminescent device as claimed in claim 11,characterized in that the liquid crystalline mixture additionallycontains at least one fluorescent dye.
 13. An electro-luminescent deviceas claimed in claim 11, characterized in that the liquid crystallinemixture additionally contains at least one polymerizable compound. 14.An electro-luminescent device as claimed in claim 13, characterized inthat the at least one polymerizable compound contains a mesogenic group.15. An electro-luminescent device as claimed in claim 11, characterizedin that an alignment layer is coated on at least one substrate.
 16. Anelectro-luminescent device as claimed in claim 11 comprising at leastone hole transport layer and at least one electron transport layer,characterized in that from the at least one hole transport layer towardthe at least one electron transport layer, at least one pair of adjacentlayers' ionization potential becomes higher and higher and/or at leastone pair of adjacent layers' energy of lowest unoccupied molecular orbitbecomes lower and lower.
 17. An electro-luminescent device as claimed inclaim 16, wherein each layer comprises at least one luminous material,characterized in that the ionization potential of the luminous materialof the at least one electron transport layer is higher than theionization potential of the luminous material of the at least one holetransport layer and/or the energy of lowest unoccupied molecular orbitof the luminous material of the at least one electron transport layer islower than the energy of lowest unoccupied molecular orbit of theluminous material of the at least one hole transport layer.
 18. Anelectro-luminescent device as claimed in claim 11, characterized in thatit contains separated pixel areas.
 19. An electro-luminescent device asclaimed in claim 18, characterized in that the separated pixel areascontain at least a two-color luminescent liquid crystalline mixture. 20.A method of preparation an electro-luminescent device as claimed inclaim 13, characterized in that the liquid crystalline mixturecontaining at least one polymerizable compound is aligned in that anelectric field is applied.
 21. A method as claimed in claim 20,characterized in that the aligned liquid crystalline mixture is fixedvia polymerization and/or crosslinking reaction.
 22. A method ofpreparation an electro-luminescent device as claimed in claim 15,characterized in that the alignment layer is formed, the formedalignment layer is aligned and the achieved alignment of the alignmentlayer is fixed via polymerization and/or cross-linking reaction.
 23. Amethod as claimed in claim 22, characterized in that on the alignmentlayer at least one additional layer is formed, which has a differentfunction than the alignment layer.